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Organic Chemistry I
LAB EXAM: FINAL
BROMINATION OF BENZENE
SYNTHESIS AND PURIFICATION OF BROMOBENZENE: PROCEDURE
DATA TABLE
Chemical Boiling point C Melting Point C Density g/mL Solubility
Benzene 80.1 5.5 0.88 Slightly in H2O
Toluene 110.6 -93 0.87 Slightly in
H2O
Bromobenzene 155-156 -30.8 1.50 Insoluble
Dibromobenzene 220.40
87.31 0.96 Insoluble
MATERIALS:
Graduated cylinder
Weight scale
Buchner funnel
Filter flask
Rubber stopper
Hot
plate
Thermometer
Conical funnel
Various size beakers
Fractionating column (for reflux)
Various sized round bottom flasks
Distillation head
Condenser
Vacuum adapter
Clamps and stands
Test tubes
PROCEDURE:
Pre Lab:
The experiment
should be carried out AWAY from the sunlight. Before beginning the experiment,
we need to make sure that the benzene used in the experiment is free of toluene
and water. To do this, we must put anhydrous calcium chloride into a flask of
benzene to "dry" it and remove the toluene. This is then decanted into a vacuum
filter, which will leave only pure benzene.
Synthesis of Bromobenzene:
Set up a reflux and place in a cold-water bath. In addition to setting up
the reflux apparatus, set up an inverted funnel gas trap. This will collect the
hydrogen bromide gas in the water and not in our lungs when the reflux takes
place. Pour 2.3 mL of benzene in the round bottom flask of the reflux apparatus.
After this, pour the iron (III) bromide (FeBr3) into the same round bottom
flask. If there is a vigorous initial reaction, wait until this has almost
stopped and then heat the water bath to a temperature of 25-30O C to initiate
the reflux. Reflux at this temperature for an hour. After an hour has passed,
raise the temperature of the water to 65-70O C, and allow refluxing for about 45
minutes. At this point, the color of the liquid should be dark. At this point,
the reaction between the benzene and the bromide molecules should be complete.
Purification of Bromobenzene:
Transfer this liquid to a separatory
funnel, and shake and vent with a 10% aqueous solution of sodium hydroxide. The
lower layer should be the bromobenzene layer. Drain this layer out into a
beaker. Add water to the liquids in the separatory funnel, and again shake and
vent to ensure the absence of bromide. Drain if necessary.
After the
extraction, the bromobenzene solution needs to be dried. Again, put anhydrous
calcium chloride into a beaker with the bromobenzene and shake. Set aside for
about 30 minutes. Decant this solution, and filter this through a vacuum filter.
Place the filtered liquid in a round bottom flask.
Set up a distillation
apparatus. Place the bromobenzene is a round bottom flask and distill into a
clean flask. Collect the distillate only from between 150-160OC. Re-distil this
liquid to obtain a more pure solution of bromobenzene. The liquid collected from
this distillation should be between 155-156OC. It should be colorless and have a
slightly "agreeable odor." This will be considered the "pure" bromobenzene and
the reaction yield should be 80%.
Purity tests:
Test 1:
Determine
the boiling point of the liquid by measuring a boiling point range during the
distillation. It should be close to 155-156OC.
Test 2:
Weigh the liquid
and measure the volume. The density should be calculated. The density of
bromobenzene is 1.50 g/mL. The calculated density should be close to this
number.
Test 3:
Run the solution through a gas chromatography machine.
Possible peaks seen in the gas chromatography could be from bromobenzene,
benzene, toluene, and dibromobenzene. This means that there could be 4 major
peaks, arranged according to boiling point. For the order, refer to the data
table above. The peak for the bromobenzene should be the largest in percent area
if this experiment has been successful.
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